Abstract

The Ruby Range is one of several uplifted Precambrian blocks in southwest Montana, near the northwestern margin of the Archean Wyoming province. It was metamorphosed during the Big Sky orogeny ~ 1.72-1.79 Ga (Baldwin et al., 2014; Cramer et al., 2013). This study seeks to characterize further the P-T metamorphic conditions of the Big Sky orogenic event as recorded in the central Ruby Range. Metapelite samples were collected during July 2014 as part of a Keck Geology Consortium summer research project. Garnet-biotite-sillimanite-bearing metapelite rocks associated with marble, amphibolite and metamorphosed banded iron formation were sampled from the Christensen Ranch Metamorphic Suite (CRMS) at the highest structural levels of the central Ruby Range along the Stone Creek drainage. Textures, mineral assemblages, mineral chemistry, and mineral properties preserve (1) evidence for multiple episodes of garnet growth during prograde metamorphism, (2) migmatitic textures indicative of partial melting and melt-loss during prograde metamorphism assumedly until peak-temperature, and (3) re-equilibrium textures formed during retrograde metamorphism. All samples contain the assemblage biotite + garnet + sillimanite + quartz, consistent with upper amphibolite to lower granulite facies metamorphism. Five samples were further investigated using petrographic observations, conventional geothermobarometry, whole rock geochemical analyses and SEM/EDS textural and mineral analyses, thermodynamic modeling and Quartz-in-garnet (QuiG) barometry to determine P-T conditions and history recorded in the metapelite samples. A peak temperature of ~800 ºC at a pressure of 8.5 kbar was determined in three samples, and an equilibration P-T value of ~700 ºC and ~ 7 kbar was determined in two samples, using thermodynamic modeling coupled with petrographic observations. Higher-pressure (peak-P) metamorphic conditions were determined in garnet grains showing multiple episodes of garnet growth. Raman spectroscopy on quartz grains included in garnet during garnet growth was used for QuiG barometry, recording peak-pressures of P ~ 10-12 kbar for temperatures between 650-800 ºC on the prograde metamorphic P-T path. Peak-pressure is reached before and at a lower temperature than peak-temperature conditions at lower pressures on the prograde path, whereas mineral assemblages and chemistry more likely record peak-T and retrograde conditions as shown by conventional geothermobarometry and thermodynamic modeling results for the five samples modeled. Modeling done assuming water saturation at the solidus as well as melt-reintegration modeling for two samples showed more consistent results for stable assemblages involving a liquid or melt phase, with temperature results consistently ~ 25ºC above ‘dry rock' or LOI modeling. Melt-reintegration modeling provided some insight into potential melt-loss or melt-retention in samples modeled. Based on these results, a P-T path for the metapelite samples from the CRMS is proposed, consistent with the P-T path and metamorphic history proposed by Cheney and others (2004a) in the adjacent Tobacco Root Mountains. These results complement previous research done on the Proterozoic Big Sky orogeny in the Tobacco Root and Highland Mountains, showing consistent Peak-T and reequilibration P-T conditions, as well as approximate peak-P conditions. Lithologies from the Tobacco Root Mountains, notably the CRMS and ICMS units, are not equivalent as CRMS ages only show Big Sky ages between 1.79-1.71 Ga whereas ICMS ages include Big Sky ages as well as older, Archean ages of ~ 2.45 Ga from a previous tectothermal event (Cheney et al., 2004b). However, the consistency of P-T results indicative of the metamorphic conditions of the Big Sky orogeny in both of these Laramide uplifted blocks reinforces the P-T constrains previously determined for the Proterozoic Big Sky orogeny.